The present invention relates to reflective type liquid crystal display devices, and more particularly to reflective type liquid crystal display devices suitable adopted for various kinds of audio-visual appliances, information display devices, and game machines, such as word processors, notebook type personal computers, electronic still cameras, portable video cameras, portable information terminus, vehicle information display devices.
FIG. 25 is a cross-sectional view showing an arrangement of a reflective type liquid crystal display device 101 of a Guest Host (GH) mode using a conventional quarter-wavelength plate. The reflective type liquid crystal display device 101 is composed of light-passing substrates 102 and 103, a liquid crystal layer 104, transparent electrodes 107 and 109, orientation films 108 and 110, a quarter-wavelength plate 111, and a reflector plate 112. It is assumed here that the liquid crystal layer 104 flanked by the light-passing substrates 102 and 103 made of glass includes dichroic dye 106, and the liquid crystal has a positive dielectric constant anisotropy.
The belt-like transparent electrodes 107 and 109 are provided on the respective surfaces 102a and 103a, of the light-passing substrates 102 and 103, facing the liquid crystal layer 104. The light-passing substrates 102 and 103 are disposed so that the transparent electrodes 107 and 109 cross at right angles. The orientation films 108 and 110 are provided on the transparent electrodes 107 and 109 provided on the light-passing substrates 102 and 103. The surfaces of the orientation films 108 and 110 receive rubbing or other treatment to orientate liquid crystal molecules 105 of the liquid crystal layer 104 parallel to the substrates. The light-passing substrates 102 and 103 are disposed so that the directions in orientation treatment of the orientation films 108 and 110 are parallel to each other. Besides, the quarter-wavelength plate 111 and the reflector plate 112 are disposed in this order on the surface 103b, of the light-passing substrate 103, facing the liquid crystal layer 104. The quarter-wavelength plate 111 is disposed so that the optic axis thereof forms an angle of 45xc2x0 to the direction in rubbing orientation treatment of the liquid crystal molecules 105.
Display in a GH mode exploits the absorption coefficient anisotropy of the dichroic dye added to the liquid crystal layer. For example, if dichroic dye having a bar structure is used, since the dye molecules have properties to become orientated parallel to the liquid crystal molecules, it becomes possible to change the orientation state of the dye molecules by changing the orientation state of the liquid crystal molecules with the application/non-application of an electric field.
For example, since the p type dye has an absorption axis, i.e. a transitional dipole moment, practically parallel to the longer axes of the molecules, the p type dye absorbs the polarization component parallel to the longer axes substantially, but absorbs little of the polarization component perpendicular thereto. The absorption coefficient for light of the dichroic dye varies, in this manner, depending upon the directions of the molecular axes, thereby effecting bright and dark display.
Here, referring to the FIGS. 26(a) and 26(b), the following description will explain the principles of the operation of the reflective type liquid crystal display device 101. The explanation will deal with the case where the p type dye is used as the dichroic dye 106 and black and white display is carried out.
FIG. 26(a) shows a state where no voltage is applied, while FIG. 26(b) shows a state where voltage is applied. In the state where no voltage is applied, the liquid crystal molecules 105 are orientated along the direction in the orientation treatment of the orientation films 108 and 110. Therefore, the dichroic dye 106 is also orientated along the direction in the orientation treatment of the orientation films 108 and 110. Part of incident light 113 on the side of the light-passing substrate 102, i.e., light 113a having the vibration plane parallel to the longer axes of the molecules of the dichroic dye 106, is absorbed by the dichroic dye 106. Other part to the light 113, i.e., light 113b having the vibration plane perpendicular to the longer axes of the molecules of the dichroic dye 106, passes through the liquid crystal layer 104. The light 113b is circularly polarized as it passes the quarter-wavelength plate 111, and is circularly polarized in the opposite direction as it is reflected by the reflector plate 112. Subsequently, the light 113b becomes light 113a having the vibration plane parallel to the longer axes of the molecules of the dichroic dye 106 as it passes the quarter-wavelength plate 111 again, and is absorbed by the dichroic dye 106. Therefore, the reflective type liquid crystal display device 101 can carry out dark display.
Meanwhile, in the state where voltage is applied, the liquid crystal molecules 105 are orientated along the direction of the electric field. Therefore, the dichroic dye 106 is also orientated along the direction of the electric field. The incident light 113, is not absorbed by the dichroic dye 106, passes through the liquid crystal layer 104, does not change the polarization state thereof as it passes through the quarter-wavelength plate 111, is reflected by the reflector plate 112, passes through the quarter-wavelength plate 111 again, and enters the liquid crystal layer 104. Since the light 113 is not absorbed by the dichroic dye 106, the light 113 exits the liquid crystal layer 104 without changing at all. Therefore, the reflective type liquid crystal display device 101 can carry out bright display.
The reflective type liquid crystal display device 101 is disclosed, for instance, by Japanese Laid-Open Patent Application No. 52-129450/1977 (Tokukaisho 52-129450). Besides, Japanese Laid-Open Patent Application No. 54-26756/1979 (Tokukaisho 54-26756) discloses technology to carry out dark display with voltage applied and bright display with no voltage applied in the arrangement of the above reflective type liquid crystal display device 101, by adding dye to Host crystal having an orientation film of slanting perpendicular orientation and having Guest Host liquid crystal of a negative dielectric constant anisotropy.
In the reflective type liquid crystal display device 101, the performance of the reflector plate 112 dictates display quality. That is, if the reflector plate used for such display principle does not preserve the polarization of incident light, the clockwise polarized light is not effectively transformed to the anti-clockwise polarized light, or vice versa, as mentioned above. As a result, light leaks during dark display, contributing to a reduction of contrast.
An example of reflector members capable of preserving polarization is a flat mirror-surface reflector member. However, the mirror-surface reflector member reflects the images of external objects at the surface thereof, and causes new problems: namely, reflections of nearby objects appear on the surface, overlapping the displayed image in the bright state, and visibility is seriously degraded. The reflector plate therefore has light diffusion characteristics as well.
A reflector plate is essential which can both preserve the light diffusion characteristics and control the polarization. Japanese Laid-Open Patent Application No. 7-218906/1995 (Tokukaihei 7-218906), as an example, discloses a reflector plate composed of a concave-convex portion made of smooth photosensitive resin and a aluminum film provided thereon. The application reports that 50% or more of the polarization should be preserved to achieve the contrast of 4 or more and that 70% or more of the polarization should be preserved to achieve the contrast of 7 or more. For these reasons, it has been impossible to obtain a reflector plate with properties that can achieve both the diffusion and the impeccable preservation of polarization.
The incident light entering the reflective type liquid crystal display device 101 passes through the liquid crystal layer 104, passes through the light-passing substrate 103, is reflected, passes through the light-passing substrate 103 again, and exits. Therefore an observer watching the display in an orthogonal direction observes double images caused by the thickness of the glass, which means that the display quality of the reflective type liquid crystal display device 101 is degraded. The double images can be eliminated to some extent by forming the reflector plate and the quarter-wavelength plate inside the liquid crystal layer. However, unless the concave-convex shapes on the above reflector plate preserving the polarization are made as small as possible, the quarter-wavelength plate varies in thickness depending upon where it is measured, resulting in a lower display quality. Japanese Laid-Open Patent Application No. 8-106087/1996 (Tokukaihei 8-106087) therefore discloses technology to form a planarization film on the concave-convex shapes of the reflector plate, which creates a new problem of an increase in the number of steps in manufacture.
Incidentally, in recent years, we have seen rapidly expanding adoption of liquid crystal display devices in various kinds of information and audio-visual display devices such as notebook type personal computers and portable television sets. Among the reflective type liquid crystal display devices, those carrying out display by reflecting ambient light that enters from outside are especially getting attention, because they require no backlight as light sources, consume less power, and can be made thin and light in weight.
Conventionally, the reflective type liquid crystal display device has adopted a liquid crystal display element of either TN (Twisted Nematic) method or STN (Super Twisted Nematic) method. However, in terms of display principles, such a liquid crystal display element needs to be flanked by a pair of polarizer plates and provided thereon with a reflector plate. This creates a problem of double image, because the thickness of the glass substrate in the liquid crystal display device causes visibility to vary depending upon the angle at which the user looks at the glass substrate, i.e. the angle of the direction at which the user looks at the liquid crystal display device to the normal of the glass substrate.
Japanese Laid-Open Patent Application No. 55-48733/1980 (Tokukaisho 55-48733) discloses a liquid crystal display device of a reflective type TN method using a single polarizer plate and a quarter-wavelength plate. The liquid crystal display device adopts a liquid crystal layer twisted by 45xc2x0 between the upper and lower substrates flanking the liquid crystal layer, and carries out bright and dark display by switching the polarizer plate of incident linearly polarized light between two states where the polarization is parallel, and makes 45xc2x0, to the optic axis of the quarter-wavelength plate by way of the controls of applied voltage. The liquid crystal display device is composed of, to name from the light-entering side, a polarizer plate, a 45xc2x0-twisted liquid crystal display element including a liquid crystal layer twisted by 45xc2x0 between the substrates flanking the liquid crystal layer, a quarter-wavelength plate, and a reflector plate.
The following description will explain a voltage control method for a display state of the aforementioned prior art.
Linearly polarized light from the polarizer plate enters the liquid crystal layer on the light-entering side, and passes through the liquid crystal layer while rotating the plane of the polarized light according to the twist of the orientation of the liquid crystal, becomes light having a vibration direction parallel to the slow axis or the advanced phase axis of the quarter-wavelength plate, and enters the quarter-wavelength plate. The linearly polarized light having a plane of polarized light parallel to the optic axis of the quarter-wavelength plate passes through the quarter-wavelength plate with the linearly polarized state being preserved, and reflected by the reflector plate. The reflected light, in the same manner as upon the entering, passes through the quarter-wavelength plate without changing at all, passes through the liquid crystal layer while rotating the plane of polarized light according to the twist of the orientation of the liquid crystal, and as a result, becomes linearly polarized light having a plane of polarized light in the same direction as upon the entering. The exiting light passes through the polarizer plate, effecting bright display.
Meanwhile, in a state where the twist is eliminated by applying voltage to the liquid crystal layer, linearly polarized light from the polarizer plate passes through the liquid crystal layer without rotating the plane of polarized light and therefore without changing at all, and enters at about 45xc2x0 with respect to the optic axis of the quarter-wavelength plate. The light then is changed in the polarization state to, for example, clockwise circularly polarized light by the quarter-wavelength plate, and enters the reflector plate. The circularly polarized light reflected by the reflector plate reverses the direction in which it proceeds, and becomes anti-clockwise circularly polarized light. The light, reflected and passing through the quarter-wavelength plate again, becomes linearly polarized light having a plane of polarized light perpendicular to the linearly polarized light that enters the quarter-wavelength plate upon the entering, and passes through the untwisted liquid crystal layer in the direction opposite to that upon the entering without being modified at all. Since the plane of polarized light is not rotated in the same manner as upon the entering, the light becomes linearly polarized light in an absorption direction of the polarizer plate, effecting dark display. In the description above, the polarization state after the incident light passes through the liquid crystal layer and the quarter-wavelength plate is clockwise circularly polarized light. However, the same effect is produced with the polarization state being anti-clockwise. In short, the prior art carries out display by modulating with the liquid crystal layer the plane of the linearly polarized light entering the quarter-wavelength plate.
A possible disposition, besides the disposition for carrying out dark display with voltage applied and bright display with no voltage applied in this manner (hereinafter, will be referred to as Normally White disposition), is to displace the placement direction of the optic axis of the quarter-wavelength plate by 45xc2x0 in order to carry out dark display with no voltage applied and bright display with voltage applied (hereinafter, will be referred to as Normally Black disposition). In such a case, the vibration direction of the linearly polarized light is transformed by the quarter-wavelength plate when no voltage is applied.
Normally Black and Normally White are the same in that a liquid crystal cell subjected to orientation of a twist angle of 45xc2x0 is disposed between a single polarizer plate and a quarter-wavelength plate, and that a single polarizer plate permits display similar to the display performed by a conventional TN type liquid crystal display device.
However, in the display principles for the conventional reflective type liquid crystal, unless the reflector plate preserves good polarization, the aforementioned transformation from clockwise circularly polarized light to anti-clockwise circularly polarized light, and vice versa, and the transformation from linearly polarized light to linearly polarized light are conducted with less efficiency, reducing contrast. So, a flat mirror-surface reflector member may be employed as a reflector plate that preserves polarization. However, as mentioned above, the mirror-surface reflector member reflects the images of nearby objects, which disrupts good viewing in bright display.
To minimize those setbacks, and to diffuse ambient light to the viewing direction as well, a stretched aluminum film for diffusing ambient light needs to be adopted as the reflector plate. However, with such a reflector plate, the aforementioned transformation from clockwise circularly polarized light to anti-clockwise circularly polarized light, and vice versa, and the transformation from linearly polarized light to linearly polarized light are conducted with less efficiency, reducing contrast.
In short, the conventional reflector plate cannot possess both good diffusing properties and good polarization preservation properties.
For example, in the liquid crystal display device disclosed in Japanese Laid-Open Patent Application No. 55-70817/1980 (Tokukaisho 55-70817) including a quarter-wavelength plate and a reflector plate on the exterior of the liquid crystal cell, light passes through different pixels in the liquid crystal layer upon entering and exiting due to the thickness of the back glass of the liquid crystal layer. This results in visibility difference depending upon the viewing angle, limiting the direction in which fine and high-quality display is possible. The present invention has an object to solve those problems and to offer a fine high-contrast reflective type liquid crystal display device free from viewing angle dependency.
In view of the problems, an object of the present invention is to offer a reflective type liquid crystal display device that possesses both good diffusing properties and good polarization preservation properties at the same time.
To achieve the above object, the present invention has a liquid crystal display element provided with:
a quarter-wavelength plate; and
a pair of substrates, each substrate being provided with a transparent electrode,
the reflective type liquid crystal display device carrying out display by using ambient light,
said element, comprising:
a reflector member, possessing an excellent polarization preservation property, for reflecting incident light; and
a complex member, composed of a complex of liquid crystal and a polymer, for selectively diffusing incident light.
Conventionally, a reflector member possessing an excellent polarization preservation property cannot be used, because the reflection of surroundings appears on the display device. By contrast, with the above arrangement, since a complex member for selectively diffusing incident light is provided, the diffusion prevents the reflection of surroundings to appear during bright display and improves visibility. Besides, since the reflector member has an excellent polarization preservation property, conventional problems of leaking light and degraded contrast caused by a poor polarization preservation property during dark display are surely eliminated. For these reasons, the present invention can offer a high contrast reflective type liquid crystal display device capable of effecting darker black display and brighter white display.
The complex member is preferably arranged to diffuse only light polarized in a particular direction while preserving the direction of polarization of the light, and used in a combination with a liquid crystal layer composed of liquid crystal and dichroic dye having a transitional dipole moment at least in a direction of a longer axis of a molecule of the liquid crystal.
In addition, the complex member is disposed between the substrate and the reflector member, and is arranged as a liquid crystal layer of liquid crystal orientated with a twist angle in a range of 40xc2x0 to 50xc2x0, and the quarter-wavelength plate is disposed between the reflector member and the liquid crystal layer.
For a fuller understanding of the nature and advantages of the invention, reference should be made to the ensuing detailed description taken in conjunction with the accompanying drawings.